Rapid Assessment Reference Condition Model
The Rapid Assessment is a component of the LANDFIRE project. Reference condition models for the Rapid Assessment were
created through a series of expert workshops and a peer-review process in 2004 and 2005. For more information, please visit
www.landfire.gov. Please direct questions to helpdesk@landfire.gov.
Potential Natural Vegetation Group (PNVG)
R5SHST
Southwestern Shrub Steppe
General Information
Contributors (additional contributors may be listed under "Model Evolution and Comments")
Modelers
Reviewers
Wendell Hann
Vegetation Type
Shrubland
Dominant Species*
PROS
ACGR
OPUN
LARR
YUCC
PLMU
BOCU
ARIST
whann@fs.fed.us
Doug Zollner
General Model Sources
Literature
Local Data
Expert Estimate
LANDFIRE Mapping Zones
25
26
dzollner@tnc.org
Rapid AssessmentModel Zones
California
Great Basin
Great Lakes
Northeast
Northern Plains
N-Cent.Rockies
Pacific Northwest
South Central
Southeast
S. Appalachians
Southwest
Geographic Range
This PNVG lies in the southwest and southern Great Plains (primarily southeastern Arizona, southern New
Mexico, and west Texas).
Biophysical Site Description
This type typically occurs on upland flats, benches, gentle slopes, and in the foothills of the desert mountain
ranges. Soils can range from shallow and rocky, to deep loamy, to heavy clay soils.
Vegetation Description
Vegetation in this system can be characterized as an open shrubland with grass dominated by mesquite,
catclaw acacia, creosote bush, tarbush, flourensia, opuntia, yucca, black grama, tobosa grass, blue grama,
sideoats grama, and various threeawn species, with intermingled forbs. This type correlates with Kuchler’s
(1964) types 58 and 59.
Disturbance Description
Naturally, this system experiences frequent, stand replacing fire occurrences that are associated with average
to above average herbaceous biomass production cycles that are related to average to above average
moisture periods. Mixed fires also may occur. A mixed fire will not kill all the shrubs due to reduced fuel
loads. The mean fire interval is approximately 10 years with high variation due to year to year deviation in
grass production related to drought and moisture cycles. Fire years are typically bimodal occurring in the
late spring (May and June) and fall (September and October) correlated with grass production following
spring and summer monsoon moisture. Removal of the fine fuels through grazing activities increases the
variation of the fire interval.
Adjacency or Identification Concerns
This ecological system is a broadly defined desert grassland, mixed shrub-succulent or xeromorphic tree
savanna that is typical of the Borderlands of Arizona, New Mexico, and northern Mexico; it extends west to
the Sonoran Desert, north to the Mogollon Rim, and eastward into west Texas throughout much of the
*Dominant Species are from the NRCS PLANTS database. To check a species
code, please visit http://plants.usda.gov.
Final Document 9-30-2005
Page 1 of 4
Chihuahuan Desert.
This PNVG is similar to the PNVG R3SHST from the Southwest model zone.
Local Data
Expert Estimate
Literature
Sources of Scale Data
Scale Description
This landscape is adequate in size to contain natural variation in vegetation, soils, and disturbance regimes.
Issues/Problems
Fire and climate are the primary factors influencing this ecological system. Drought and lack of fire tend to
increase invasive woody species and reduce the herbaceous component. Impacts of historic grazing by
buffalo may not have played a significant impact in this system in Arizona and New Mexico. Invasive
species such as burrow weed (Isocoma tenuisecta) and broom snakeweed (Gutierrezia sarothrae) can take
advantage of cool-season precipitation and dominate on disturbed sites; pricklypear and cholla (Opuntia
spp.) can also dominate on disturbed sites and outcompete herbaceous species thereby reducing fuel
continuity and reduce the controlling effects of fire.
Model Evolution and Comments
Compare information with NRCS ecological site descriptions; ask for review by NRCS Plant Materials
Specialist located at the Tucson Plant Materials Center, 520-292-2999: (Bruce Munda bruce.munda@az.usda.gov) and NRCS Rangeland Specialist Dan Robinett (dan.robinett@az.usda.gov).
Ask for review by U of A professors: George Ruyle and Mitch McClaran. Contact range professors at New
Mexico State for review.
Succession Classes**
Succession classes are the equivalent of "Vegetation Fuel Classes" as defined in the Interagency FRCC Guidebook (www.frcc.gov).
Class A
10 %
Early1 All Struct
Description
Dominant Species* and
Canopy Position
PLMU3
BOCU
ARIST
PROSO
Mid-Upper
Mid-Upper
Mid-Upper
Upper
This Class is dominated by
resprouts of desert grassland
Upper Layer Lifeform
species and post-fire associated
Herbaceous
forbs and half-shrubs. This Class
Shrub
typically exists where fires have
burned relatively hot (replacement
Tree
fire severity) in Classes B and C.
Succession in this Class can
quickly progress to either Class B
or Class C, depending on soil types.
Fuel Model 1
Class B
5%
Mid1 Closed
Description
Greater than 15 percent shrub
cover and 30-50 percent grass and
forb cover; generally associated
with more productive soils. Effects
of cumulative drought can cause a
shift from this class to Class C.
Successional progression from
Dominant Species* and
Canopy Position
PROSO Upper
ACGR Upper
YUCCA Upper
PLMU3 Lower
Upper Layer Lifeform
Herbaceous
Shrub
Tree
Fuel Model
Structure Data (for upper layer lifeform)
Min
10 %
Cover
Height
Max
30 %
Herb Short <0.5m
Tree Size Class
Shrub Short 0.5-0.9m
no data
Upper layer lifeform differs from dominant lifeform.
Height and cover of dominant lifeform are:
The dominant lifeform at this successional
stage is primarily comprised of various
herbaceous grass and forb species (both annual
and perennial).
Structure Data (for upper layer lifeform)
Min
15 %
Cover
Height
Max
40 %
Shrub Medium 1.0-2.9m
no data
Shrub Tall >3.0 m
Tree Size Class
Upper layer lifeform differs from dominant lifeform.
Height and cover of dominant lifeform are:
1
*Dominant Species are from the NRCS PLANTS database. To check a species
code, please visit http://plants.usda.gov.
Final Document 9-30-2005
Page 2 of 4
Class A to this Class occurs on
deep, productive soil types.
Surface fires can maintain this
Class. Mixed severity fires can
move this system to Class C.
Native grazing may have had
minimal impact on this landscape.
Class C
85 %
Dominant Species* and
Canopy Position
PROSO
ACGR
PLMU3
BOCU
Mid1 Open
Description
Upper
Upper
Lower
Lower
Less than 15 percent shrub cover
and 20 to 40 percent grass and forb
cover generally associated with less Upper Layer Lifeform
productive cobbly and gravelly
Herbaceous
soils. Successional progression
Shrub
from Class A to this Class occurs
Tree
on dry, less productive soil types.
Fuel Model 1
Class D
0%
Dominant Species* and
Canopy Position
Late1 All Structu
Structure Data (for upper layer lifeform)
Min
5%
Cover
Height
Shrub Medium 1.0-2.9m
Tree Size Class
Structure Data (for upper layer lifeform)
Tree Size Class
Upper Layer Lifeform
Herbaceous
Shrub
Tree
Fuel Model
Class E
0%
Min
0%
Max
0%
no data
no data
no data
Upper layer lifeform differs from dominant lifeform.
Height and cover of dominant lifeform are:
no data
Dominant Species* and
Canopy Position
Late1 All Structu
Structure Data (for upper layer lifeform)
Min
Cover
Description
Height
%
no data
Tree Size Class no data
Upper Layer Lifeform
Herbaceous
Shrub
Tree
Fuel Model
Shrub Tall >3.0 m
no data
Upper layer lifeform differs from dominant lifeform.
Height and cover of dominant lifeform are:
Cover
Height
Description
Max
15 %
Max
%
no data
Upper layer lifeform differs from dominant lifeform.
Height and cover of dominant lifeform are:
no data
Disturbances
*Dominant Species are from the NRCS PLANTS database. To check a species
code, please visit http://plants.usda.gov.
Final Document 9-30-2005
Page 3 of 4
Disturbances Modeled
Fire
Insects/Disease
Wind/Weather/Stress
Native Grazing
Competition
Other:
Other
Historical Fire Size (acres)
Avg: 1000
Min: 25
Max: 5000
Sources of Fire Regime Data
Literature
Local Data
Expert Estimate
Fire Regime Group: 2
I: 0-35 year frequency, low and mixed severity
II: 0-35 year frequency, replacement severity
III: 35-200 year frequency, low and mixed severity
IV: 35-200 year frequency, replacement severity
V: 200+ year frequency, replacement severity
Fire Intervals (FI)
Fire interval is expressed in years for each fire severity class and for all types of
fire combined (All Fires). Average FI is central tendency modeled. Minimum and
maximum show the relative range of fire intervals, if known. Probability is the
inverse of fire interval in years and is used in reference condition modeling.
Percent of all fires is the percent of all fires in that severity class. All values are
estimates and not precise.
Avg FI
Replacement
Mixed
Surface
All Fires
Min FI
Max FI
Probability
12
37
0.08333
0.02703
9
0.11037
Percent of All Fires
76
24
References
Brown, James K.; Smith, Jane Kapler, eds. 2000. Wildland fire in ecosystems: effects of fire on flora. Gen.
Tech. Rep. RMRS-GTR-42-vol. 2. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky
Mountain Research Station. 257 p.
Kuchler, A. W. 1964. Manual to accompany the map of potential natural vegetation of the conterminous
United States. American Geographical Society. Spec. Publ. No. 36. Lib. Congress Cat. Card Num. 64-15417.
156 p.
NatureServe. 2005. International Ecological Classification Standard: Terrestrial Ecological Classifications.
Terrestrial ecological systems of the South Central US:DRAFT legend for Landfire project. NatureServe
Central Databases. Arlington, VA. Data current as of 13 January 2005.
Schmidt, Kirsten M, Menakis, James P., Hardy, Colin C., Hann, Wendel J., Bunnell, David L. 2002.
Development of coarse-scale spatial data for wildland fire and fuel management. Gen. Tech. Rep. RMRSGTR-87. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research
Station. 41 p. + CD.
U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory
(2002, December). Fire Effects Information System, [Online]. Available: http://www.fs.fed.us/database/feis/.
*Dominant Species are from the NRCS PLANTS database. To check a species
code, please visit http://plants.usda.gov.
Final Document 9-30-2005
Page 4 of 4